I just built new packs for my DIMMs and it wasn't too bad.
spot welder:
https://www.amazon.com/gp/product/B09FJQN7DT
batteries:
https://www.18650batterystore.com/products/panasonic-ncr18650ga-ga6
wrap:
https://www.amazon.com/gp/product/B097ZPZMNG
Disassemble the original battery by cutting off the outer wrap.
Remove any tape.
Slide the top cardboard cover off the pcb.
Desolder the blue wire.
Desolder the pcb from the two tabs
Ensure you've got the two bumpers on either side of the pcb. If they fell off be sure to glue them back on.
Cut the cardboard where the pcb was to split the batteries apart and get the holder.
Orient the new batteries in series. Use the holder and tape them together so its easier to work with.
Tack a nickel strip across one end.
Tack a thinner strip to that strip and bend it around for the blue wire.
Fashion some new tabs for the pcb out of the nickel strip and tack them to the other ends of the batteries. There's a decent amount of wiggle room with the pcb for this so it doesn't have to be perfect.
Solder the blue wire to the middle tab and the pcb to the top tabs. Make sure you have positive on positive and negative on negative.
Put the cardboard cap back on the pcb and use too much kapton tape to wrap it all up.
Finally squeeze the heat shrink wrap over the whole thing and cautiously use a heat gun to shrink it.